The suspect, who entered the European Union country illegally in late 2015, is "in police custody and proceedings are under way to send him back (to Pakistan)," an immigration official dealing with the case told AFP.
The man will be banned from re-entering Romania for 10 years, the official added.
In a statement released late yesterday, secret service officials said the man had drawn attention after spreading "online propaganda backing several terrorist groups active in his home region, supporting Islamist extremism, and calling for global jihad".
The Romanian government is seeking to tighten its anti- terror legislation following deadly Islamist attacks in France and Belgium.
On Monday, Romanian lawmakers introduced a parliamentary bill aimed at making it easier to expel foreign citizens suspected of extremist activities.